This invention relates to a rock bit with a built-in stabilizer on the bit body that can contact the wall of a borehole without unduly disrupting fluid flow or generating elevated temperatures in the adjacent bit body.
Heavy-duty drill bits or rock bits are employed for drilling wells in subterranean formations for oil, gas, geothermal steam, and the like. Such rock bits have a body connected to a drill string and generally three hollow cutter cones mounted on the body for drilling rock formations. Each cutter cone occupies a major part of a 120.degree. sector of the bit. The cutter cones are mounted on steel journals or pins integral with the bit body at its lower end. In use, the drill string and rock bit body are rotated in the borehole, and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the borehole being drilled.
Each cutter cone has a number of generally circular rows of inserts or cutting elements. In some rock bits the cones have hardened steel teeth integral with the cone, which may also be coated with a hardfacing material. Many cones have cemented tungsten carbide inserts forming the cutting elements. As the cone rotates, the inserts of each row are applied sequentially in a circular path on the bottom of the borehole in the formation being drilled. As the cutter cones roll on the bottom of the borehole the teeth or carbide inserts apply a high compressive load to the rock and fracture it. The cones may be skewed from a radial direction to force some "skidding" action. The cutting action in rolling cone cutters is typically by a combination of crushing and chipping the rock formation.
In operation, a rock bit is attached to the lower end of a hollow drill string that extends from the ground surface to the rock bit at the bottom of a borehole being drilled. The drill string is rotated by the drill rig at the ground surface (or sometimes a downhole motor is used) which rotates the drill bit around it's longitudinal axis on the bottom of the borehole. Thus, the rolling cutter cones are caused to rotate and as weight is applied to the bit by the weight of the drill string, the carbide inserts in the cones crush, chip, gouge, and scrape the formation to dislodge chips of rock. Drilling fluid is pumped downwardly through the drill string and rock bit, returning to the surface via the annular space between the drill string and the wall of the borehole being drilled. The particles of rock formation dislodged by the bit are carried out of the borehole by drilling fluid. The drilling fluid also cools the bit.
The tungsten carbide inserts along the periphery of a bit that is nearest the base of the cones and which define the diameter of the hole being drilled are known as gage inserts. As the rolling cutter cones rotate, the gage inserts engage rock at the periphery (or gage) of the hole being drilled to dislodge rock formation. The gage inserts are most susceptible to wear because they undergo both abrasion and compression as they scrape against the gage of the borehole. Appreciable wear on the gage inserts is undesirable because this may result in an undersize borehole. When a replacement drill bit is inserted toward the bottom of an undersized borehole, the replacement bit may pinch against the hole wall and cause premature wear of the gage inserts and overload of the bearings between the rock bit body and cutter cones.
The cones on a rock bit are, therefore, commonly provided with a circular row of inserts adjacent to the base of the cone known as heel row inserts. The cones are angled so that the faces of the heel row inserts define the gage of the rock bit.
The cutter cones are mounted on journal pins extending downwardly and inwardly from a leg portion of the rock bit body. The lowermost portion of the leg, which is the largest diameter portion of the rock bit, is rounded and relatively thin where it covers the base of the cone. The exterior of the bit body has a curved face which has come to be known as the shirttail. This name derives from the curved lower edge of the face adjacent to the cone. Recessed channels extend longitudinally along the bit body towards the pin end between the shirttail portions. The shirttail portion of the rock bit body may be bare steel or the lower edge may have a layer of hardfacing deposited thereon to minimize wear due to rubbing of the shirttail against the wall of the borehole.
The drill string has a smaller diameter than the borehole being drilled. This, of course, creates a certain amount of angularity to the drill string which may be imparted to the rock bit itself. If the rock bit tilts, even though the angle may be very small, there can be excessive pressure of the lower portions of the bit against the rock formation as the bit is rotated. This may cause undue wear of the shirttail.
Stabilizers are often mounted in the drill string above the rock bit for minimizing the tilting of the rock bit. A stabilizer is a sub having a diameter close to the gage of the borehole to keep the drill string centered. Preferably, the use of such stabilizer subs is to be avoided.
Many years ago it was decided to form stabilizer pads integral with the rock bit body an appreciable distance above the bottom of the shirttail. Such an integral stabilizer is described and illustrated in U.S. Pat. No. 3,628,616, for example. The stabilizer pad on the rock bit body was a significant advance that helped maintain the direction of drilling and minimize undue wear on the shirttail.
The integral stabilizer pad may be a raised portion of steel forged integral with the rest of the bit body. A stabilizer pad may also be a piece of steel welded onto the bit body or a pad of steel built up with weld metal which is then machined or ground to a desired final shape. The pad may be steel coated with hardfacing for wear resistance or a separate pad of hardfacing material may be brazed to the steel body. Such a stabilizer pad may have flat cemented tungsten carbide inserts which bear against the gage of the borehole and stabilize the bit.
Although the stabilizer pad on the bit body was recognized as a significant advance and has been adopted for many models of drill bits, some of its shortcomings have been recognized, particularly in recent years when rock bits have been operated at higher rotational speeds. Heating of the rock bit body as a consequence of friction between the stabilizer pad and borehole wall may become significant.
The cutter cones mounted on the rock bit body are lubricated by a viscous grease which is filled within a space around the cone bearings. Pressure and temperature variations in the rock bit environment may limit the ability to seal the grease in and seal abrasive drilling fluid out. Many modern rock bits are, therefore, provided with a pressure compensated grease reservoir in an upper portion of the bit body for maintaining grease at the bearing surfaces. Unfortunately, the stabilizer pads are adjacent the grease reservoir and heating may reduce the viscosity of the grease, thereby reducing its capability for lubricating the bearing surfaces. Even without a grease reservoir, it is undesirable to have excessive temperatures generated.
Part of the heating problem is due to the stabilizer pad. Heat is carried away from the rock bit by the drilling fluid flowing upwardly through the annulus between the rock bit body and the wall of the borehole. A drilling pad bearing against the wall of the borehole leaves no room for circulation of drilling fluid and extraction of heat. This can be exacerbated by packing of particles around the stabilizer pad, which further inhibits flow of drilling fluid.
Excess heat may also deteriorate the rubber boot in the grease reservoir and its failure may lead to rapid failure of the rock bit when the bearings are no longer properly lubricated.
A problem sometimes occurs with stabilizer pads that are welded onto the body instead of forged integral with the body. The welding to build up the body or add a steel pad may produce a stress riser below the pad as well as damaging the metallurgical properties of the steel. This has actually resulted in breakage of the legs of the bit. This not only disrupts drilling, but the resultant junk can be costly to fish or mill from the borehole. Most such failures come from welded on pads or built-up pads.
The stabilizer pads also act somewhat like paddles rotating in the borehole, which disrupt upward flow of fluid which carries away the particles of rock produced by drilling. The disrupted fluid flow may cause abnormal packing of the reservoir cap with formation that may prevent the grease compensation reservoir from functioning or may dislodge the reservoir cover cap from the bit, both of said conditions will lead to premature bearing failure.
Integral stabilizer pads are commonly made with sloping upper and lower faces, however, abrasion commonly causes the taper to wear away, leaving a sharp ledge, particularly at the lower edge of the stabilizer pad. Due to the vagaries of drilling rock bits sometimes temporarily drill an offset or oversize hole. After an episode of such drilling a small shoulder may be formed in the wall of the borehole. When the stabilizer pads encounter the shoulder, they may hang up on the shoulder and retard drilling. In severe cases bits may get stuck when tripping into a hole. This problem is common enough that there are experienced drillers that refuse to use bits with stabilizer pads.
It would therefore be desirable to eliminate the stabilizer pad. However, at the same time it is desirable to maintain the enhanced stability. Satisfaction of these countervailing desiderata is provided in practice of this invention.